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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.08.021 Byline: Darrell F. Strobel Keywords: Pluto; Pluto; atmosphere; Aeronomy; Atmospheres; dynamics; Atmospheres; structure Abstract: Hydrodynamic escape of N.sub.2 molecules from Pluto's atmosphere is calculated under the assumption of a high density, slow outflow expansion driven by solar EUV heating by N.sub.2 absorption, near-IR and UV heating by CH.sub.4 absorption, and CO cooling by rotational line emission as a function of solar activity. At 30 AU, the N.sub.2 escape rate varies from (4-6.4)x10.sup.26 moleculess.sup.-1 in the absence of heating, but driven by an upward thermal heat conduction flux from the stratosphere, for lower boundary temperatures varying from 70-100 K. With solar heating varying from solar minimum to solar maximum conditions and a calculated lower boundary temperature, 88.2 K, the N.sub.2 escape rate range is (1.8-6.7)x10.sup.27 moleculess.sup.-1, respectively. LTE rotational line emission by CO reduces the net solar heat input by at most 35% and plays a minor role in lowering the calculated escape rates, but ensures that the lower boundary temperature can be calculated by radiative equilibrium with near-IR CH.sub.4 heating. While an upward thermal conduction heat flux at the lower boundary plays a fundamental role in the absence of heating, with solar heating it is downward at solar minimum, and is, at most, 13% of the integrated net heating rate over the range of solar activity. For the arrival of the New Horizons spacecraft at Pluto in July 2015, predictions are lower boundary temperature, T.sub.0[approximately equal to]81 K, and N.sub.2 escape rate [approximately equal to]2.2x10.sup.27 moleculess.sup.-1, and peak thermospheric temperature [approximately equal to]103 K at 1890 km, based on expected solar medium conditions. Author Affiliation: Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218-2687, USA Article History: Received 21 March 2007; Revised 20 July 2007

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.02.008 Byline: Conor Laver (a), Imke de Pater (a), Henry Roe (b), Darrell F. Strobel (c) Keywords: Io; Atmospheres; composition; Jupiter; satellites; Volcanism Abstract: We report observations of the ro-vibronic a.sup.1[DELTA][right arrow]X.sup.3[SIGMA].sup.- transition of SO at 1.707 [mu]m on Io. These data were taken while Io was eclipsed by Jupiter, on four nights between July 2000 and March 2003. We analyze these results in conjunction with a previously published night to investigate the temporal behavior of these emissions. The observations were all conducted using the near-infrared spectrometer NIRSPEC on the W.M. Keck II telescope. The integrated emitted intensity for this band varies from 0.8x10.sup.27 to 2.4x10.sup.27photons/s, with a possible link to variations in Loki's infrared brightness. The band-shapes imply rotational temperatures of 550-1000 K for the emitting gas, lending further evidence to a volcanic origin for sulfur monoxide. An attempt to detect the B.sup.1[SIGMA][right arrow]X.sup.3[SIGMA].sup.- transition of SO at 0.97 [mu]m was unsuccessful; simultaneous detection with the 1.707 [mu]m band would permit determination of the SO column abundance. Author Affiliation: (a) Department of Astronomy, 601 Campbell Hall, University of California, Berkeley, CA 94720, USA (b) Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA (c) Department of Earth and Planetary Sciences and Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA Article History: Received 28 March 2006; Revised 6 February 2007

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.12.015 Byline: Darrell F. Strobel Keywords: Titan; Tides; atmospheric; Atmospheres; dynamics; Atmospheres; structure; Satellites; atmospheres Abstract: Tidal waves driven by Titan's orbital eccentricity through the time-dependent component of Saturn's gravitational potential attain nonlinear, saturation amplitudes (|T.sup.'|10K, |u.sup.'|20ms.sup.-1, and |w.sup.'|5cms.sup.-1) in the upper atmosphere ([greater than or equal to]500 km) due to the approximate exponential growth as the inverse square root of pressure. The gravitational tides, with vertical wavelengths of [approximately equal to]100-150 km above 500 km altitude, carry energy fluxes sufficient in magnitude to affect the energy balance of the upper atmosphere with heating rates [greater than or equal to]10.sup.-9ergcm.sup.-3s.sup.-1 in the altitude range of 500-900 km. Author Affiliation: EPS/Olin 121, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218-2687, USA Article History: Received 9 June 2005; Revised 16 December 2005

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.01.016 Byline: Xun Zhu (a), Darrell F. Strobel (b) Keywords: Titan; Atmospheric dynamics; Meteorology Abstract: Titan's atmospheric winds, like those on Venus, exhibit superrotation at high altitudes. Titan general circulation models have yielded conflicting results on whether prograde winds in excess of 100 mas.sup.-1 at the 1 mbar level are possible based on known physical processes that drive wind systems. A comprehensive two-dimensional (2D) model for Titan's stratosphere was constructed to systematically explore the physical mechanisms that produce and maintain stratospheric wind systems. To ensure conservation of angular momentum in the limit of no net exchange of atmospheric angular momentum with the solid satellite and no external sources and sinks, the zonal momentum equation was solved in flux form for total angular momentum. The relationships among thermal wind balance, meridional circulation, and zonal wind were examined with numerical experiments over a range of values for fundamental input parameters, including planetary rotation rate, radius, internal friction due to wave stresses, and net radiative drive. The magnitude of mid-latitude jets is most sensitive to a single parameter, the planetary rotation rate and results from the conversion of planetary angular momentum to relative angular momentum by the meridional circulation, whereas the strength of meridional circulation is mainly determined by the magnitude of the radiative drive. For Titan's slowly rotating atmosphere, the meridional temperature gradient is vanishingly small, even when the radiative drive is enhanced beyond reasonable magnitudes, and can be inferred from zonal winds in gradient/thermal wind balance. In our 2D model large equatorial superrotation in Titan's stratosphere can be only produced through internal drag forcing by eddy momentum fluxes, which redistribute angular momentum within the atmosphere, while still conserving the total angular momentum of the atmosphere with time. We cannot identify any waves, such as gravitational or thermal tides, that are sufficiently capable of generating the required eddy forcing of >50 mas.sup.-1aTitan-day.sup.-1 to maintain peak prograde winds in excess of 100 mas.sup.-1 at the 1 mbar level. Author Affiliation: (a) The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723-6099, USA (b) Departments of Earth and Planetary Sciences and Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA Article History: Received 10 February 2004; Revised 20 January 2005

We present a model that describes Io's delayed electrodynamic response to a temporal change in Io's atmosphere. Our model incorporates the relevant physical processes involved in Io's atmosphere-ionosphere-magnetosphere electrodynamic interaction to predict the far-ultraviolet (FUV) radiation as Io enters Jupiter's shadow and re-emerges into sunlight. The predicted FUV brightnesses are highly nonlinear as the strength of the electrodynamic interaction depends on the ratios of ionospheric conductances to the torus Alfven conductance, but the former are functions of electrodynamics and the atmospheric density, which decays rapidly upon entering eclipse. Key factors governing the time evolution are the column density due to sublimation and the column density due to volcanoes, which maintain the background atmosphere during eclipse. The plasma interaction does not react instantaneously, but lags to a temporarily changing atmosphere. We find three qualitatively different scenarios with two of them including a post-eclipse brightening. The brightness ratio of in-sunlight/ in-eclipse coupled with the existence of a sub-jovian equatorial spot constrains the volcanic column density to several times [10.sup.18] [m.sup.-2], based on the currently available observations. Thus in sunlight, the sublimation driven part of Io's atmosphere dominates the volcanically driven contribution by roughly a factor of 10 or more. Keywords: Io's atmosphere; Io in eclipse; Satellite plasma interaction; Post-eclipse brightening

We have reanalyzed the Voyager 1 UVS solar occultations by Titan to expand upon previous analyses and to resolve inconsistencies that have been noted in the scientific literature. To do so, we have developed a detailed model of the UVS detector and improved both the data reduction methods and retrieval techniques. In comparison to the values previously determined by Smith et al. (1982, J. Geophys. Res. 87, 1351-1359) we find [N.sub.2] densities that are 25-60% higher, C[H.sub.4] densities that are smaller by a factor of 3-7, and [C.sub.2] [H.sub.2] densities that are roughly two orders of magnitude smaller. Our values for the thermospheric temperature are 153-158 K, which are approximately 20-40 K colder than previous estimates. We also report the first-ever determination from Voyager UVS data of density profile information [C.sub.2][H.sub.4], HCN, and H[C.sub.3]N Finally, we present a simple engineering model that is consistent with our new results in the upper atmosphere and merges smoothly with the model of Yelle et al. (1997, in: HUYGENS Science, Payload and Mission, in: ESA SP, vol. 1177, pp. 243-256) in the lower atmosphere. Our results provide improved constraints for photochemical models and offer scientists a better understanding of Titan's upper atmosphere as we head into the Cassini era in the exploration of the saturnian system. Keywords: Titan; Occultations; Aeronomy; Atmospheres, structure

The Io plasma torus, composed of mostly heavy ions of oxygen and sulfur, is sustained by an Iogenic mass loading rate of [~10.sup.30] amu [s.sup.-1] = 1.6 x [10.sup.28] S[O.sub.2] [s.sup.-1] or approximately [10.sup.3] kg [s.sup.-1] (A.L. Broadfoot et al., 1979, Science 204, 979-982). We argue on the basis of available power sources, reanalysis of F. Bagenal (1997, Geophys. Res. Lett. 24, 2111-2114), HST UV remote sensing, and detailed model calculations that at most 20% of this mass leaves Io in the form of ions, i.e., [less than or equal to] 3 X [10.sup.27] x ([n.sub.e,0]/3600 [cm.sup.-3]) ions [s.sup.-1], where [n.sub.e,0] is the average torus electron density. For the Galileo spacecraft Io pass in December 1995, the ion mass loading rate was [less than or equal to] 3 X [10.sup.27] ions [s.sup.-1], whereas for the Voyager epoch with lower [n.sub.e,0] (=2000 [cm.sup.-3]), this rate would be [less than or equal to] 1.7 X [10.sup.27] ions [s.sup.-1], consistent with the D.E. Shemansky (1980, Astrophys. J. 242, 1266-1277) mass loading limit of [less than or equal to] 1 x [10.sup.27] ions [s.sup.-1]. We investigate the processes that control Io's large scale electrodynamic interaction and find that the elastic collision rate exceeds the ionization/pickup rate by at least a factor of 5 for all atmospheric column densities considered ([10.sup.16]-[10.sup.21] [m.sup.-2]) and by a factor of ~100 for the most realistic column density. Consequently, elastic collisions are mostly responsible for Io's high conductances and thus generate Io's large scale electrodynamic interaction such as the generation of Io's electric current system and the slowing of the plasma flow. The electrodynamic part of Io's interaction is thus best described as an ionosphere-like interaction rather than a comet-like interaction. An analytic expression for total electron impact rates is derived for Io's atmosphere, which is independent of any particular model for the 3D interaction of toms electrons with its atmosphere.

We report the discovery of the forbidden electronic [a.sup.1] [DELTA] [right arrow] [X.sup.3] [[SIGMA].sup.-] transition of the SO radical on Io at 1.7 [micro]m with the W. M. Keck II telescope on 24 September 1999 (UT), while the satellite was eclipsed by Jupiter. The shape of the SO emission band suggests a rotational temperature of ~1000 K; i.e., the gas is extremely hot. We interpret the observed emission rate of ~2 x [10.sup.27] photons [s.sup.-1] to be caused by SO molecules in the excited [a.sup.1] [DELTA] state being directly ejected from the vent at a thermodynamic quenching temperature of ~1500 K, assuming a SO/S[O.sub.2] abundance ratio of ~0.1 and a total venting rate of ~[10.sup.31] molecules [s.sup.-1] (Strobel and Wolven 2001, Astrophys. Space Sci. 277, 1-17). The shape of our complete (1.6-2.5 [micro]m) spectrum suggests that the volcano Loki contains a small (~2 [km.sup.2]) hot spot at 960 [+ or -] 12 K, as well as a larger (~50 [km.sup.2]) area at 640 [+ or -] 5 K. Key Words: Io; infrared observations; volcanism; atmosphere composition; atmosphere structure.

Ultraviolet spectra of Titan obtained with the Faint Object Spectrograph of the Hubble Space Telescope in October 1991 and August 1992 have yielded a disk-average geometric albedo of 0.02-0.044 from 1800-3300 [Angstrom]. These results are in excellent agreement with previous UV measurements from 2200-3300 A spanning two decades, but have a significantly higher signal to noise ratio and spectral resolution. We add [approximately]400 [Angstrom] of new spectral information from 1800-2200 [Angstrom], a wavelength region in which Titan has not been detected previously. The albedo above 2200 [Angstrom] is a factor of 1.18 higher in August 1992 than October 1991, which is most likely due to the 4.3 [degrees] difference in Titan's phase angle between the two dates. Below 2200 [Angstrom] the albedo decreases only modestly with decreasing wavelength and does not show unambiguous evidence for discrete spectral features characteristic of several of the known hydrocarbons, particularly acetylene ([C.sub.2][H.sub.2]). Comparison of the albedo with best-fit models provides constraints on the optical properties and distribution of the small particle component of Titan's haze and on the [C.sub.2][H.sub.2] mole fraction. The small particle haze seems to be darker in the UV than the 'Titan' tholins analog produced in the laboratory, and the vertical extent limited to altitudes above [approximately]120-150 km. The acetylene distribution is consistent with the Voyager IRIS determination in the 130- to 180-km altitude range, where the mole fraction is of order 2 x [10.sup.-6]. Key Words: atmospheres, structure; abundances, atmospheres; Titan; satellites of Saturn.

We present an updated model for the photochemistry of Io's atmosphere and ionosphere and use this model to investigate the sensitivity of the chemical structure to vertical transport rates. S[O.sub.2] is assumed to be the dominant atmospheric gas, with minor molecular sodium species such as [Na.sub.2]S or [Na.sub.2]O released by sputtering or venting from the surface. Photochemical products include SO, [O.sub.2], S, O, Na, NaO, NaS, and [Na.sub.2]. We consider both 'thick' and 'thin' S[O.sub.2] atmospheres that encompass the range allowed by recent HST and millimeter-wave observations, and evaluate the possibility that [O.sub.2] and/or SO may be significant minor dayside constituents and therefore likely dominant nightside gases. The fast reaction between S and [O.sub.2] limits the column abundance of [O.sub.2] to [approximately][10.sup.4] less than that calculated by Kumar (J. Geophys. Res. 87, 1677-1684, 1982; 89(A9), 7399-7406, 1984) for a pure sulfur/oxygen atmosphere. If a significant source of Na[O.sub.2] or [Na.sub.2]0 were supplied by the surface and mixed rapidly upward, then oxygen liberated in the chemical reactions which also liberate free Na would provide an additional source of [O.sub.2]. Fast eddy mixing will enhance the transport of molecular sodium species to the exobase, in addition to increasing the vertical transport rate of ions. Ions produced in the atmosphere will be accelerated by the reduced corotation electric field penetrating the atmosphere. These ions experience collisions with the neutral gas, leading to enhanced vertical ion diffusion. The dominant ion, [Na.sup.+], is lost primarily by charge exchange with [Na.sub.2]O and/or [Na.sub.2]S in the lower atmosphere and by diffusion through the ionopause in the upper atmosphere. The atmospheric column abundance of SO, [O.sub.2], and the upper atmosphere escape rates of Na, S, O, and molecular sodium species are all strong functions of the eddy mixing rate. Most atmospheric escape, including that of molecular sodium species, probably occurs from the low density 'background' S[O.sub.2] atmosphere, while a localized high density 'volcanic' S[O.sub.2] atmosphere can yield an ionosphere consistent with that detected by the Pioneer 10 spacecraft.

A radiative-conductive model for the vertical thermal structure of Pluto's atmosphere is developed with a non-LTE treatment of solar heating in the C[H.sub.4] 3.3 [[micro]meter] and 2.3 [[micro]meter] bands, non-LTE radiative exchange and cooling in the C[H.sub.4] 7.6 [[micro]meter] band, and LTE cooling by CO rotational line emission. The model includes the effects of opacity and vibrational energy transfer in the C[H.sub.4] molecule. Partial thermalization of absorbed solar radiation in the C[H.sub.4] 3.3 and 2.3 [[micro]meter] bands by rapid vibrational energy transfer from the stretch modes to the bending modes generates high altitude heating at sub-microbar pressures. Heating in the 2.3 [[micro]meter] bands exceeds heating in 3.3 [[micro]meter] bands by approximately a factor of 6 and occurs predominantly at microbar pressures to generate steep temperature gradients [approximately]10-20 K [km.sup.-1] for p > 2/[micro]bar when the surface or tropopause pressure is [approximately]3 [micro]bar and the C[H.sub.4] mixing ratio is a constant 3%. This calculated structure may account for the 'knee' in the stellar occultation lightcurve. The vertical temperature structure in the first 100 km above the surface is similar for atmospheres with Ar, CO, and [N.sub.2] individually as the major constituent. If a steep temperature gradient [approximately]20 K [km.sup.-1] is required near the surface or above the tropopause, then the preferred major constituent is Ar with 3% C[H.sub.4] mixing ratio to attain a calculated ratio of T/M(= 3.5 K [amu.sup.-1]) in agreement with inferred values from stellar occultation data. However, pure Ar and [N.sub.2] ices at the same temperature yield an Ar vapor pressure of only [approximately]0.04 times the [N.sub.2] vapor pressure. Alternative scenarios are discussed that may yield acceptable fits with [N.sub.2] as the dominant constituent. One possibility is a 3 [micro]bar [N.sub.2] atmosphere with 0.3% C[H.sub.4] that has 106 K isothermal region (T/M = 3.8 K [amu.sup.-1]) and [approximately]8 K [km.sup.-1] surface/tropopause temperature gradient. Another possibility would be a higher surface pressure [approximately]10 [micro]bar with a scattering haze for p > 2 [micro]bar. Our model with appropriate adjustments in the C[H.sub.4] density profile to Triton's inferred profile yields a temperature profile consistent with the UVS solar occultation data (Krasnopolsky, V. A., B. R. Sandel, and F. Herbert 1992. J. Geophys. Res. 98, 3065-3078.) and ground-based stellar occultation data (Elliot, J. L., E. W. Dunham, and C. B. Olkin 1993. Bull. Am. Astron. Soc. 25, 1106.).

A newly developed radiative-thermal conduction model deals with the vertical structure of Io's atmosphere and predicts the presence of mesopause in the atmosphere of Io. The model is developed by the application of the SO2 absorption lines in the near infra-red and ultraviolet bands consistent with the solar heating in the atmosphere. The mesopause in the atmosphere of Io is prominent when the pressure on the surface of Io is more than 10 nanobars.

An upper limit on the tropopause argon mixing ratio is a function of the tropopause CH4 mixing ratio f(CH4). This is inferred by an analysis of the Voyager 1 Ultraviolet Spectrometer solar occultation data, and a spectrum of the North Polar Region dayglow obtained by the Titan flyby. The upper limit varies from 0.15 to 0.08. The mean molecular mass is 0.17 for any f(CH4).

Ultraviolet spectrometer stellar and solar data obtained by the Voyager 2 deep space probe were analyzed to determine the smooth thermospheric temperature and heat source characteristics of Uranus. The results showed that the temperatures indicated by the occultations were much higher than can be explained by solar extreme ultraviolet heating. However, the effects of constant temperature on pressure surfaces at low altitudes cannot be excluded.